Arrayed waveguide gratings (AWG) have recently found wide scale use in optical networks with the introduction of wavelength division multiplexing (WDM) as a means for increasing the available bandwidth for optical communications. AWGs multiplex and demultiplex multiple channels, each carried on a different wavelength into/out of a single waveguide or fiber, and can also be used in router, add-drop and switch configurations.
An AWG is very similar to well known bulk diffraction gratings except that it is a planar device, in which the waveguides of the array act as the grates of the grating. The imaging properties of an Awg are determined by a few parameters, including the lengths of the arrayed waveguides and their effective indices, resulting in phase relationships between the fields propagating in them. Any imperfection introduced during the fabrication of an AWG, such as a variation in waveguide dimensions along their length or changes in material composition will adversely affect the performance of the AWG. For example, random variations due to the fabrication process create deviations from the ideal phase needed at the far end of the waveguides and cause what can be called “phase errors.”
Most commercial AWGs are fabricated in silica on silicon, with the waveguides having large dimensions to match the modes of a fiber. These AWGs are relatively unaffected by the fabrication problems mentioned above because any fluctuations in waveguide dimensions are small compared to their size, and improved processes have helped control the uniformity of the materials composing the waveguides. However most silica AWGs use temperature control of the whole chip, to make sure they match standard channels like the ITU grid. Such temperature controlled chips are very expensive.
Recent advances in the use of standard CMOS processes for the fabrication of optoelectronic integrated circuits promises to dramatically reduce the size and cost of devices like AWGs. However, as the sizes of integrated circuits and waveguides are reduced, they become much more sensitive to fabrication issues.